Gain control circuits



Oct. 31, 1939.

R. BLAIR GAIN CONTROL CIRCUITS Filed Sept. 8, 19:58

2 Sheets-Sheet 1 Law I- IMPEDANCE.

HEAT IPAPIDLY L COOL JLOWLY X /N VE N TOR RRBLA/R BY A T TOR/VE V Oct. 31 1939. R. R. BLAIR GAIN CONTROL CIRCUITS 2 Sheets-Sheet 2 Filed Sept. 8, 1938 26 a4 2220 l8 l6 l4 l2 IO RF INPUT/N 0a mwmmm INVENTOR 1?. R. BLAIR ATTORNEY Patented Oct. 31, 1939 ATENT oF-Fics GAIN CONTROL CIRCUITS Boyer R. Blair, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application September 8, 1938, Serial No. 229,000

11 Claims.

' This invention relates to gain control circuits and particularly to gain control circuits for governing the energy level of signals on transmission lines.

One object of the invention is to provide gain control circuits having an oscillator operating on a steeply inclined cut-off portion of its characteristic curve that shall govern an amplifier on a signal transmission line in an improved manner.

Another object of the invention is to provide a signal transmission line with gain control circuits having an oscillator operating on a steeply inclined cut-off portion of its characteristic curve that shall control the oscillator for producing an output which builds up slowly and reduces quickly to control an amplifier in the transmission line.

In a long transmission line it is necessary to provide repeaters or amplifiers at intervals along the line and to control the amplifiers to maintain the energy level of the signals on the transmission line substantially constant. The energy level of the signals on a transmission line may be controlled by governing an impedance in the transmission line or by governing an amplifier in the transmission line. The gain control circuits may be governed according to the energy level of currents carried by the transmission line.

The invention has been illustrated by means of a carrier current system having a pilot current for governing the operation of gain control circuits. The gain control circuits are governed according to the energy level of a pilot current for controlling an amplifier of the type disclosed in the patent to H. S. Black No. 2,102,671, December 21, 1937.

In one form of the invention the beta circuit of an amplifier in the transmission line is controlled by gain control circuits according to the energy level of the pilot current on the transmission line beyond the amplifier. Aresistance element which has a high temperature coefiicient of resistance and which may be composed of silver sulphide is connected across the beta circuit of the amplifier. The heating of the silver sulphide element is controlled by the gain control circuits.

The gain control circuits comprise a filter connected to the transmission line beyond the amplifier therein to select the pilot current. The selected pilot current is amplified by a pentode tube which not only serves as an amplifier but also, as a rectifier of (SO-cycle power current. Regulated GO-cycle power current is supplied not only to the anode of the pentode tube but also to the screen grid thereof. Rectified alternating current in the anode circuit of the pentode tube is employed as grid bias for the pentode tube and plate current for an oscillator.

A second space discharge device comprising a control grid, a cathode and two anodes serves not only as a rectifier but also as an oscillator. This second space discharge device may be a pentode tube, if so desired, with the screen grid and the suppressor grid connected together to form one of the anodes. The amplified pilot current supplied by the amplifier tube is rectified by the rectifier portion of the second space discharge device. The rectified potential serves to impress negative potential on the grid in the oscillator portion of the second space discharge device. The oscillator portion of the second space discharge device operates with an inputoutput characteristic curve having a steeply inidly and to cool relatively slowly. This varia- 0 tion in the heating and. cooling of the silver sulphide element has a tendency to cause the gain control circuits to hunt. In order to counteract this tendency to hunt when silver sulphide elements of certain types areemployed, the osa cillator portion of the second space discharge device in the gain control circuits is constructed so that the output from the oscillator builds up slowly and reduces fairly quickly.

In some cases it may be desirable to have the amplifier tube of the gain control circuits supplied with alternating current bias from the regulated voltage. When the amplifier tube is supplied with alternating current bias it is found that the gain control circuits are less sensitive to power or voltage variations, particularly if the variations are rapid. I

In the accompanying drawings:

Fig. 1 is a diagrammatic view of gain control circuits constructed in accordance with the invention;

Figs. 2 and 3 are diagrammatic views illustrating modifications of the gain control circuits shown in Fig. 1;

Fig. 4 is a curve diagrammatic showing the characteristic curve of the control circuits shown in Figs. 1, 2 and 3.

Referring to Fig. 1 of the drawings, an amplifier I of the feedback type is shown on a transmission line, having input conductors 2 and 3a and output conductors 3 and 4. The amplifier I may be of the type disclosed in the abovementioned patent to H. S. Black No. 2,102,671, December 21, 1937. The amplifier I is operated by means of three tubes 5, 6 and I, which may be of any suitable type. The beta circuit 8 of the amplifier I has a shunt impedance 9 connected across it. A portion of the shunt impedance 9 comprises a resistance element I!) having a high negative temperature coemcient of resistance. The resistance element I is heated by a coil II under control of gain control circuits I2. The transmission line carries not only carrier currents which may be of radio frequency but also a pilot current. The pilot current controls the operation of the gain control circuits I2.

The gain control circuits comprise a filter I3 of any suitable type which is connected across the conductors 3 and 4 by means of a potentiometer M. The filter I3 selects the pilot current on the transmission line. The potentiometer I4 serves as a sensitivity control for the gain control circuits. Two space discharge devices I and I5 are provided in the gain control circuits I2 which are controlled by the pilot current output from the filter I3. The space discharge device I5 is in the form of a pentode tube having a control grid IT, a cathode I8, a screen grid I9, a suppressor grid 20 and an anode 2|. The tube I5 serves not only as an amplifier of the selected pilot current but also as a low frequency rectifier. The second space discharge device I6 is shown in the form of a pentode tube having a control grid 22, a cathode 23, a screen grid 24, a suppressor grid 25 and an anode 26. The screen grid 24 and the suppressor grid 25 are shown connected together to substantially form an additional anode. If so desired the space discharge device I6 may be formed with two anodes, a cathode and a control grid.

A source of alternating current 2! is provided for supplying power to the pentode tube I5. The source 21 is directly connected to a transformer 28 which in turn is connected to a transformer '29 through a ballast lamp 30. The ballast lamp 353 connected to the secondary winding of the transformer 28 insures a substantially constant current supply for the transformer 29. The transformer 29 should be designed to keep both core and copper losses low. The transformer 29 may supply filament current to both the space discharge devices I 5 and It as well as plate and screen grid potential to the pentode tube 15. The connections of the filaments for the cathodes of the devices IEand I8 to the secondary winding 3| of the transformer 29 have been omitted from the drawings for the sake of simplicity.

The anode circuit for the pentode tube I5 may be traced from one terminal of the secondary winding 32 for the transformer 2!! through inductance 33 and capacity 34 in parallel, anode 2!, cathode I8, resistance elements '35 and 36, resistance element 31 and capacity 38 in parallel and shunted by the primary winding of transformer 48, grids 24 and 25, and cathode 23, and ground return to the other terminal of the secondary winding 32. The capacity 34 and the inductance 33 serve to tune the circuit of the pentode tube I5 to the frequency of the pilot current. The potential drop across the resistance element 35 serves to provide potential bias for the control grid H. A resistance element 39 which is connected between the control grid i7 and the junction of the resistance elements 35 and 36 serves as a grid lead. A low frequency filter of the power current on the pentode tube 5 is provided by means of the resistance ele ments 35 and 35 in combination with the condenser 38 and a condenser 40.

The output from the pentode tube i5, which is the radio frequency pilot current plus Gil-cycle modulation products, is impressed between the plate 26 and cathode 23 of the second space discharge device IG. Rectification of the pilot current is effected and the rectified potential is impresed across a resistance element 4!. The potential drop across the resistance M is employed to govern the oscillator portion of the space discharge device ifi. The input circuit to the oscillator portion of the space discharge device It may be traced from one terminal of the resistance AI through an inductance 12, a resistance 43, a resistance i l, the grid 22 and the cathode 23 to the other terminal of the resistance 35 The unwanted modulation products which would greatly dull the cut-off of the oscillator portion of the tube It are removed by means of the condenser 45. Condensers 46 and 61 in the gain control circuits I2 serve as blocking condensers.

he output from the oscillator portion of the space discharge device It is connected by a transformer 46 to the heater coil ii. The output circuit of the oscillator, which includes the primary winding of the transformer 43, may be traced from one terminal of the primary winding of the transformer so through grids 24 and 25, cathode 23, and condenser shunted by the resistance element .17, to the other terminal of. the primary winding for the transformer 48. The voltage drop across the resistance 3? supplies plate potential to the grids 26 and 25, which in the oscillator portion of the device It serve as an anode. The voltage drop across the resistance 37 is obtained from the anode circuit of the space discharge device I5.

The feedback necessary for the oscillation of the pentode I5 is obtained from a potentiometer Sill connected across the secondary winding of the transformer 58. The potentiometer 59 may also be employed to control the output of the gain control circuits and to some extent the performance of the gain control circuit. Reduced feedback and output results in a sharper cut-ofi' and also higher plate current By reason of this fact the output from the oscillator should be fairly close to the desired value by impressing the proper voltage from the transformer 29 on the anode of tube l5 rather than by reducing the feedback of the potentiometer 50 upon the control grid 22 of the oscillator. The feedback on the control grid 22 from the potentiometer 5D is effected by means of a condenser 5i. The radio frequency output circuit from the pentode amplifier tube I5 may be traced from the anode 2i through the condenser 34 shunted by the inductance 33, by-pass condenser 53, and condenser 4E1 to the cathode I8. The diode or rectifier portion of the tube I6 comprising the anode 26 and the cathode 23 is coupled to the output circuit of the tube I5 by means of the condenser 45.

A copper oxide rectifier 54 and a condenser 55 are connected across the input circuit of the oscillator portion of the device I6 for controlling the building up and reducing of the oscillator output. The copper oxide rectifier 54 and 2 of the drawings are connected to the output the 'condenser 55,: which are connected between the cathode 23 and the-control grid 22, serve to insurethat the oscillator'output builds up relatively slowly and reduces relatively fast. This action on thepart of the oscillator is only necessary in case certain types-of resistance elementsare employed inth'e beta circuit of the transmission line amplifier. Certain types of silver sulphide elements have the tendency to heat very rapidly and to cool fairly slowly.

-Unless some means is taken to counteract this action on the part of the silver sulphide element, hunting on the part of the gain-control circuits will take place. Slow-building up of the output'from the oscillator and fast reduction in the output of the oscillator will compensate for the rapid heating-andslow cooling of a copper oxide element. 1 i

In the system above describedassume that the signal currents on the conductors 3 and 4 beyond the amplifier I are reduced below normal value and that the'energy value of the pilot current is reduced at the same time. The pilot current I selected by thefilter I3.is impressed across the input circuit of the pentode tube I5. The amplified pilot current is then rectified by the diode portion of the device l6 comprising anode 26 and cathode 23. The rectified pilot current is then impressed on thev resistance element 4! for coni.

trolling the' oscillator portion of the device I6.

Inasmuch as the energy .value of the pilot current is assumed to be reduced, then the potential drop across the resistance element M is reduced. Consequently, the negative bias on the gridZ-Z is reduced to increase the'oscillator output impressed on the transformer. The output from the-oscillator is increased. slowly. An" .increased current is then supplied by the transformer 48 to the heater coil II. The temperature of the resistance element II] is then increased to decrease the resistance thereof and lower the feedback through the beta circuit 8 to increase the output from the amplifier I. g

' In case the energy level of the signals" on the transmission line beyond the. amplifier I go abovenormal value, 'the pilot current also goes coil I I and consequently reduces the tempera-.

ture of the resistance element I0. This increases the resistance value of the element III to increase the feedback through the beta circuit 8. This in turn reduces the output from the'line amplifier I. In the above manner the output from the amplifier I is maintained substantially constant. 1 I i Referring to Fig. 2 of the drawingsi'a modi fication of the circuit shown in Fig. .1 is illustrated wherein alternating current bias is impressed on thegrid of the first amplifier tube.

Like parts'in the circuits shown in Fig. 2 to those shown in Fig. 1 have been indicated by similar reference characters. The conductors 56 in Fig.

conductors 3 and 4, and conductors 51 are connected tothe heater coil II. The portion of the circuits shown in Fig. 1 above the clot and .dash lineXX may be applied to the circuit shown in Fig. 2.

Bias for the control grid Ilof tube I in the circuit shown in Fig. 2 is not obtained from the drop across the resistance element 35, as in the circuit shown in Fig. 1 but is obtained from the secondary winding 32 of the transformer 29. The secondary winding 32 of, the transformer 29 isdivided into two sections 58 and 59. A potentiometer 60 which is connected. across the transformer section 59 is connected through the grid leak 39 to the control grid I1. The screen grid I9 is connected between the sections 58 and 59 of the secondary winding 32, as in the circuit shown in Fig, 1. p p

The secondary winding 3I supplies current of the order of 6.3 volts to the filaments of zthe spacedischarge devices I5 and I6. The second-' ary winding 32 supplies potential of the order of 400 volts to the anode 2I and approximately a potential of 250 volts is supplied to the screen grid I9. However, theabove voltages do not appear entirely across the first space discharge device since the cathode I8 is approximately 1 10 volts off ground. The potentiometer lill is varied in order to control the regulated output. When operating the first amplifier tube I5 with alternating current 5,v the amplifier was found to be less sensitive to power voltage variations and particularly to rapid fluctuations. This circuit, aside from the bias on the tube I5, operatesin exactly the same manner as the circuit shown in Fig. 1 with one exception. Provision is not provided inthe circuit of Fig. 2- for operating the oscillator part of the device I6. to build up the output slowly and to reduce the output fairly fast. It is to be understood, however, that the copper oxide element 54, the condenser and the resistance elements 43 and '44 may be added to the circuit shown in Fig. 2, if desired. The oscillator portion of the device I6 operates with the characteristic output-input curve having a steeply inclined cut-oil portion the same. i

as the oscillator shown in Fig. l. The charac teristic input output curve of the oscillator portion of the device I6 hasa steeply inclined cutofi portion similar to the steeply inclined portion. of the curve shown in Fig. 4 of the drawings.

heater coil. The steeply inclined cut-off portion of the input-output characteristic curves of the oscillators shown in Figs. 1, 2 and 3 is similar I to the steeply inclined portion of the curve shown in Fig. 4.

The circuit shown in Fig. '3 of the drawings is a modification of the circuit shown in Fig. 1

of the drawings and like parts. will be indicated by similar reference characters. The conductors 56in Fig. 3 of thedrawings may be connected,

to the output conductors 3 and 4, and the con-,

ductor's 57 may be connected to theheater coil, I I".-

In the circuit shown in Fig. 3 of the drawings,

' the source of alternating power 27 is connected to a transformer which in turn is coupled to a transformer H by means of a condenser E2. The core of the transformer H is saturated and the condenser 72 tunes the circuit of the two transformers to the frequency of the power current. The tuning effect, while permitting large primary currents to flow, results in a substantially constant output from the secondary winding of the transformer TI. The secondary winding of the transformer H is similar to the secondary winding 32 of the transformer 29 shown in Fig. 1 of the drawings and is connected to like parts of the circuit. The system shown in Fig. 3 of the drawings operates in the same manner as the system shown in Fig. 1 of the drawings except that no'provision is made for making the output from the oscillator portion of the device l6 build up slowly and reduce fairly fast. It is to be understood that the resistance element 43, condenser 55 and the copper oxide rectifier 54 shown in the circuits of Fig. l of the drawings may be added to the circuit of Fig. 3 of the drawings, if so desired.

Modifications in the circuits and in the arrangement and location of parts may be made within the spirit and scope of the invention and such modifications are intended to be covered by the appended claims.

What is claimed is:

1. A transmission line carrying signal and pilot currents and having an amplifier therein, a space discharge device serving as an oscillator and a rectifier, means for filtering and amplifying the pilot current on the transmission line, means 38 for rectifying the amplified pilot current by the rectifier portion of said device, means to adjust theoscillator portion of said device to have a characteristic output-input curve with a steeply inclined cut-off portion and for operating the oscillator portion of the device on the steeply inclined portion of the characteristic curve, means for operating the oscillator portion of said device according to the output from the rectifier portion and means controlled by the oscillatory output for governing the amplifier in the transmission line to maintain the amplifier output constant.

' 2. A transmission line carrying signal and pilot currents and having an amplifier therein, a space discharge device serving as an oscillator and a rectifier and comprising a control grid, a cathode ard two anodes, means for filtering and emplifying the pilot current on the transmission line, means for rectifying the amplified pilot current by the rectifier portion of said device, means for adjusting the oscillator portion of said device to have a characteristic output-input curve with a steeply inclined cut-off portion and for operating the oscillator portion of the device on the steeply inclined cut-off portion of the characteristic curve, means for impressing negative potential from said rectified pilot current on the grid of said device to control the oscillatory output, and means controlled by the oscillatory output for governing the amplifier in the transmission line to maintain theamplifier output constant;

- 3. A transmission line carrying signal and pilot v currents and having an amplifier therein, a filter amplifying the selected pilot current, means for operating said tube with alternating anode potential, a space discharge device serving as as oscillator and a rectifier and comprising a control grid, a cathode and two anodes, means for supplying the anode of the oscillator portion of said device with rectified potential from the anode circuit of said tube, means for adjusting the oscillator portion of said device to have a characteristic curve with a steeply inclined cut-off portion and for operating the oscillator portion of said device on the steeply inclined cut-off por tion of the characteristic curve, means for rectifying the amplified pilot current by the rectifier portion of said device'for supplying a negative,

bias to the grid of said oscillator portion of said device to control the oscillator output, and means controlled by the oscillator output for governing the amplifier in the transmission line to maintain the amplifier output constant.

i. Atransmission line carrying signal and pilot currents and having an amplifier therein, a pentode tube serving as an amplifier, a space discharge device serving as as oscillator and a rectifier, the oscillator portion of said device operating with a characteristic curve having a steeply inclined cut-01f portion and operating on the steeply inclined cut-oifportion of the characteristic curve, means for amplifying the pilot current by the amplifier pentode tube, means for supplying alternating anode potential to said pentode tube, means for supplying alternating plate potential rectified by said tube as anode potential to the oscillator portion of said device, means comprising the rectifying portion of said device for rectifying the amplified pilot current from said tube and for impressing negative potential from said rectifiedpilot current on the grid of the oscillator portion of said device to control the oscillator output, and means controlled by the oscillatory output for governing the amplifier in the transmission line to maintain the amplifier output constant.

5. A transmission line'carrying signal and pilot currents and having an amplifier therein, a filter for selecting said pilot current from the line beyond the amplifier therein, an amplifier tube for amplifying the selected pilot current, a source of regulated alternating current power supplied to the amplifier tube, a space discharge device serving as an oscillator and a rectifier and comprising a control grid, a cathode and two anodes, means for supplying the anode of the oscillator portion of said device with rectified potential from the anode circuit said amplifier tube, means for rectifying the amplified pilot current by the rectifier portion of said device to supply potential to the grid of the oscillator portion of said device for controlling the oscillator output, and means controlled by the oscillator output for governing the amplifier in the transmission line to maintain the amplifier output constant.

6. A transmission line carrying signal and pilot currents and having an amplifier therein, a filter for selecting said pilot current from the line beyond the amplifier therein, an amplifier tube comprising an anode, a cathode and a control grid for amplifying the selected pilot current, a source of alternating current for supplying anode potential to said amplifier tube, means for coupling said source to the anode of the amplifier tube and comprising two transformers one of which has a saturated core and a condenser between the secondary winding of the first transformer and the primary winding of the second transformer for tuning the transformers to the frequency of the alternating current, a space discharge device comprising a grid, two anodes and a cathode and serving as an oscillator and a rectifier, means for supplying the oscillator portion of said device with rectified anode potential from the anode circuit of said amplifier tube, means for rectifying the amplified pilot current by the rectifier portion of said device to supply potential to the grid of the oscillator portion of said device to control the oscillator output and means controlled by the oscillator output for governing the amplifier in the transmission line to maintain the amplifier output constant.

7. A transmission line carrying signal and pilot currents and having an amplifier therein, a space discharge device serving as an oscillator and a rectifier, means for filtering and amplifying the pilot current on the transmission line, means for rectifying the amplified pilot current by the rectifier portion of said device, means for controlling the oscillator portion of said device according to the rectified potential supplied by the rectifier portion of said device, means for controlling the oscillator portion of said device to build up the output slowly and to reduce the output relatively fast, and means controlled according to the oscillator output for governing the amplifier in the transmission line to maintain the amplifier output constant.

8. A transmission line carrying signal and pilot currents and having an amplifier therein, means comprising a resistance element of silver sulphide for controlling said amplifier, said resistance element having the characteristic of heating rapiclly and cooling slowly, an oscillator space discharge device, means for adjusting said oscillator to have an output-input characteristic curve with a steeply inclined cut-01f portion, means for controlling said oscillator according to the energy level of the pilot current on the transmission line, means for operating the oscillator to build up the output slowly and to reduce the output relatively fast, and means for heating said element of silver sulphide according to the oscillator output to maintain the output of the transmission line amplifier substantially constant.

9. A transmission line carrying signal and pilot currents and having an amplifier therein, means comprising a resistance element having a high negative coefficient of resistance for controlling said amplifier, said resistance element having the characteristic of heating rapidly and cooling slowly, a space discharge oscillator having a control grid, means for impressing potential on said grid according to the energy level of said pilot current to control said oscillator and effect a more than proportional change in the oscillator output upon pilot current change, means for controlling said oscillator to build up the output slowly and to reduce the output relatively fast, and means for heating said resistance element according to the oscillator output to maintain the output of the transmission line amplifier substantially constant.

10. A controlled circuit having a characteristic thereof maintained substantially constant, a space discharge oscillator having a control grid, means for impressing a variable potential on said grid according to the characteristic of said circult, means for operating said oscillator on a steeply inclined portion of the output-input characteristic curve thereof when controlled by the potential impressed on the grid, means for controlling said oscillator to build up the output slowly and to reduce the output relatively fast, and means controlled according to the output from said oscillator for maintaining the characteristic of the controlled circuit substantially constant.

11. A signal transmission line having an amplifier therein, a space discharge oscillator having a control grid, means for impressing potential on said grid according to the energy level of the signals for controlling said oscillator to effect a more than proportional change in the oscillator output upon signal energy change, means comprising capacity and copper oxide elements connected across the input circuit of said oscillator for controlling the oscillator to build up the output slowly and to reduce the output relatively fast, and means controlled according to the oscillator output for governing the amplifier in the transmission line to maintain the amplifier output constant.

ROYER R. BLAIR. 

